One way of enhancing network performance is to utilize signals from multiple sensors or antennas in the network. There are a variety of existing techniques for exploiting signals from multiple sensors or antennas, especially in relation to the uplink in a cellular network.
Advanced antenna solutions such as adaptive antenna systems and MIMO (Multiple-Input Multiple-Output) systems can be used to enhance system performance. Space-time coding as represented by references [1], [2] and [3] can be regarded as a method for providing diversity for a wireless fading channel using multiple transmit and/or receive antennas. FIG. 1 schematically illustrates an example of a classical MIMO system, where a transmitting node 10 has multiple m transmit antennas and a receiving node 20 has multiple n receive antennas. In matrix form, the channel model can be expressed as:y=Hx+w x=G(ci, . . . , cp),where y is the received signal vector, H is a n by m complex channel matrix, x is the transmitted signal vector, w is a vector representation of white noise, G is a code matrix and c is a symbol in a code book, and p is the number of symbols per block. The complex channel gain matrix H can be written as:
  H  =      [                                        h            11                                    …                                      h                          1              ⁢                                                          ⁢              m                                                            ⋮                          ⋱                          ⋮                                                  h                          n              ⁢                                                          ⁢              1                                                …                                      h            nm                                ]  where hij is the complex channel gain from transmit antenna j in the transmitting node to receive antenna i in the receiving node.
Soft handover is an entirely different method of exploiting so-called multi-sensor information, now further up in the network at a combining point and based on information from multiple base stations. In soft handover, the signal from a mobile terminal is received by two or more base stations, which transfer respective decoded data to an RNC (Radio Network Controller) for combining.
Softer handover in WCDMA refers to the situation when a mobile terminal is in the overlapping coverage area of two or more adjacent sectors of a base station, where the signal from the mobile is received by each sector, and then transferred to the same RAKE receiver for maximum ratio combining.
In practice, WCDMA (Wideband Code Division Multiple Access) normally employs a rather “hard” handover known as macro selection diversity rather than ideal soft handover. This typically means that some quality or reliability indicator, such as a CRC checksum, received pilot signal strength or a frame reliability indicator, is used for enabling dynamic selection of the better data and/or frame from the base stations.
FIG. 2 schematically illustrates uplink diversity in a WCDMA system, in which a mobile terminal 10 establishes radio links with multiple base stations (or node Bs) 20-1 and 20- and/or sectors simultaneously. Softer handover, also referred to as intersector diversity, here involves the reception of signals from the mobile terminal at different sectors within the same base station 20 followed by maximum ratio combining (MRC) on soft baseband signals in the MRC combiner 22 prior to channel decoding in the channel decoder 24.
Soft handover, also referred to as intercell site diversity, typically involves the transmission of hard decision data after channel decoding, together with associated reliability information, from multiple base stations 20-1 and 20-2 to the RNC (Radio Network Controller) 30 for per-user selection combining of the decoded data according to the reliability information, for example as described in reference [4].
Ideal soft handover operates on soft baseband signals that are transferred from the base stations to a combining point for maximum ratio combining or similar combining per-user (when noise and interference from different base stations are uncorrelated), for example as described in references [5] and [6].
Reference [5] presents an uplink protocol based on the multiple-to-one relationship between base stations and mobile. As illustrated in FIG. 3, the uplink protocol involves transferring non-decoded quantized information from a number of receiving base stations 20-1 and 20-2 to a so-called controlling base station 20-3. The controlling base station 20-3 then employs majority combining, maximum ratio combining or maximum probability combining of the received quantized information for optimal decoding of the mobile 10.
Reference [6] is also related to the multiple-to-one relationship between base stations and mobile, and concerns the situation of several base stations receiving a signal from a mobile terminal and forwarding information to a central exchange node for decoding of the mobile.
Common to all known soft handover is that per-user combining is employed and that interference from other mobile terminals is generally treated as unstructured noise, thus failing to optimally reflect and consider the actual situation at the receiving base stations.